In addition to information provided by previous poster, you may look into well known link redundant/bakup dual ISP through backup interface, although this architecture does not privides load balancing you may still achive inbound traffic through backup isp link via two nat statemets, you would need atleast pix code 7.x as this feature was introduced in code 7.

Personaly I would prefer load balancing, and nat at the router like previous poster said, but it all depends what your requirements are and where all your NAT statements configurations reside and how many they currenly are in your PIX. If you are looking for just backup ISP then look into backup interface and dual ISP in PIX/ASA.